Fluoride containing chemistries have been used for many years to clean prime silicon wafers (wafers that have not yet undergone ion implantation or device construction) in the semiconductor industry. Normally, the fluoride chemistry (usually dilute hydrofluoric acid) is used as the last process step in the sequence called “RCA rinses”. The substrate is often contaminated from previous process steps with monolayer amounts of metal, anions and/or organic contaminants or surface residues (particles). These contaminants have been shown to have significant impact on the electrical integrity of simple test device structures and they need to be efficiently cleaned without impairing their integrity. Such cleaning methods could include techniques discussed in the technical literature, for example, mt. Conf. On Solid State Devices and Materials, 1991, pp. 484-486 or Kujime, T. et al., Proc. of the 1996 Semi. Pure Water and Chemicals, pp. 245-256 and Singer, P. Semi International, p. 88, October 1995.
Patents that teach methods for cleaning prime wafers with low pH solutions include U.S. Pat. Nos. 5,560,857; 5,645,737; 5,181,985; 5,603,849; and 5,705,089.
Using fluoride chemistries (usually HF) as a final RCA cleaning step will cause the silicon wafer surface to be in a hydrophobic state (the surface is covered with Si-H groups) which will repel water. During this cleaning step a certain proportion of the wafer surface is dissolved (removed). Unless the cleaning conditions are carefully monitored (time, temperature, solution composition) the substrates can be damaged, as reported by Rafols, C. et al., J. Electroanalytic Chem. 433, pp. 77-83, 1997. Numerous compositions combine water and organic solvents. The water concentration in these HF solutions is very critical. Silica oxide has an etch rate of 21 Å/min (@25° C.) in HF/water, but in isobutanol the rate was reduced to 2.14 Å/min and even lower in acetone (an aprotic solvent) the rate was only 0.12 Å/min.
After the Front End of Line (FEOL) cleaning process the wafer proceeds to the typical Back End of Line (BEOL) manufacturing process for a semiconductor devices, in which the devices might be dynamic random access memories (DRAMs), static random access memories (SRAMs), logic, electrically programmable read only memories (EPROMs), complementary metal on silicon (CMOS), and the like. Etching fabrication technology using chemical reactions (liquid or plasma) has been used as a method of forming a wiring structure on such semiconductor substrates.
A photoresist film is deposited on the wafer to form a mask, then a substrate design is imaged on the film layer, baked, and the undeveloped image is removed with a photoresist cleaner. The remaining image is then transferred to the underlying material (either a dielectric or metal) with reactive etching gases promoted with plasma energy. The etchant gases selectively attack the unprotected area of the substrate. Liquid etching chemistries, usually containing fluoride chemistries have been used extensively over the years to etch metals (Al) and dielectrics. The fluoride chemistries can be very aggressive and can result in isotropic etching (etching equally in all directions). Isotropic etching effects cannot be tolerated with today's needs for tight critical dimension control, though there have been attempts to control the isotropic etch through statistical process control techniques, as reported by Taylor, D., Solid State Technology, July 1998, p. 119.
The usual plasma etching process involves anisotropic (unidirectional) etching while at the same time the byproducts (e.g., composed of photoresist, etching gasses and etched materials) are deposited on the sidewall of etched openings as residues.
A disadvantage of forming this protective sidewall deposit is that it can be very difficult to remove the residue after the etching procedure. If the components in these residues are not removed or neutralized in some manner then the residues will absorb moisture and form acidic species that can corrode the metal structures. The resultant acid corrodes wiring materials to bring about an adverse effect such as an increase in electrical resistance and wire disconnection.
Such problems frequently occur, in particular in aluminum and aluminum alloys generally used as wiring material. The wafer substrate in contact with acidic materials, if not controlled, can destroy the metal structures.
Following completion of the etching operation it is necessary that the resist mask be removed from the protective surface to permit finishing operations. It is desirable to develop an improved cleaning composition to remove the organic polymeric substance from a coated inorganic substrate without corroding, dissolving or dulling the metal circuitry or chemically altering the wafer substrate.
Cleaning compositions used for removing photoresist coatings if not already ashed and other substrates have for the most part been highly flammable, generally hazardous to both humans and the environment, and comprise reactive solvent mixtures exhibiting an undesirable degree of toxicity. Moreover, these cleaning compositions are not only toxic, but their disposal is costly since they might have to be disposed of as a hazardous waste. In addition, these compositions generally have severely limited bath life and, for the most part, are not recyclable or reusable.
Side wall residues have been removed with either acidic organic solvents or alkaline organic solvents. The acidic solvents are generally composed of phenolic compounds or chlorinated solvents and/or an aromatic hydrocarbon and/or an alkylbenzenesulfonic acid. These formulations generally need to be used at temperatures up to and beyond 100° C. These chemistries normally need to be rinsed with isopropanol.
Dilute hydrofluoric acid solutions can under certain conditions remove the sidewall polymers by aggressively attacking the via sidewall of the dielectric and therefore changing the dimensions of the device, as taught by Ireland, P., Thin Solid Films, 304, pp. 1-12 (1997), and possibly the dielectric constant. Previous chemistries that contain HF, nitric acid, water, and hydroxylamine are aggressive enough to etch silicon, as taught by U.S. Pat. No. 3,592,773 issued to A. Muller. Recent information also indicates that the dilute HF solutions can be ineffective for cleaning the newer fluorinated hydrocarbon etch residues, as taught by K. Ueno et al., “Cleaning of CHF3 Plasma-Etched SiO2/SiN/Cu Via Structures with Dilute Hydrofluoric Acid Solutions,” J. Electrochem. Soc., vol. 144, (7) 1997. Contact holes opened on to the TiSi2 have also been difficult to clean with HF solutions since there appears to be an attack of the underlying TiSi2 layer. There may also be difficulty with mass transport of the chemicals in the narrow hydrophilic contact holes, as taught by Baklanov, M. R. et al., Proc. Electrochem. Soc., 1998, 97-35, pp. 602-609.
The photoresist around the contact hole of common interlayer dielectrics, TEOS (tetraethylorthosilicate) and boron phosphosilicate glass (BPSG), which are commonly used in ultra large scale integration (ULSI) structures for better conformity of step coverage, is usually removed with HF solutions. It is not uncommon for the HF to also attack the dielectric material. Such attack is not desirable (see Lee, C. and Lee, 5, Solid State Electronics, 4, pp. 921-923 (1997)).
The alkaline organic solvents for post etch residue removal can be composed of amines and/or alkanolamines and/or neutral organic solvents. These formulations generally must be used at temperatures above 100° C. Recently a new class of post etch residue cleaning chemistries has been used to clean these substrates. These chemistries include hydroxylamine, amines, alkanolamines and corrosion inhibitors and generally operate at temperatures of at least 20 to 30 degrees lower.
Recently, fluoride-based chemistries have been used in limited cases to remove post etch residues and to a limited extent, photoresist residues from integrated circuit substrates during BEOL (Back End of Line) processes. Many of the wafer cleaning compositions contain fluoride components, specifically hydrogen fluoride. In addition, these compositions might contain strong caustic chemicals (choline-derivatives, tetraalkyl-ammonium hydroxide, or ammonium hydroxide) such as disclosed in U.S. Pat. Nos. 5,129,955; 5,563,119; or 5,571,447 (the disclosures of which are incorporated by reference), or might use a two-phase solvent system, which contains one phase with hydrofluoric acid and water while a second phase contains a nonpolar organic solvent (ketones, ethers, alkanes or alkenes; U.S. Pat. No. 5,603,849, the disclosure of which is hereby incorporated by reference hereto). Other formulations include hydroxylamine and ammonium fluoride (U.S. Pat. No. 5,709,756, issued to Ward, the disclosure of which is hereby incorporated by reference hereto). Additional examples include quaternary ammonium salt and fluoride based compositions, as disclosed in published European Application No. 0662705, and organocarboxylic ammonium salt or amine carboxylate and fluoride-based compositions, as disclosed in U.S. Pat. No. 5,630,904, the disclosure of which is hereby incorporated by reference hereto.
Some chemistries have also included chelating agents to help remove ionic and anionic contamination from the wafer surface (International Publication No. PCT/US98/02794), but chelating agents such as citric acid, gallic acid, and catechol, among others, can be aggressive toward the aluminum oxide that covers the Al metal lines. Studies by Ohman and Sjoberg show that the strong complexing ability of citric ions can increase the aluminum oxide solubility and thereby expose the metal to further corrosion, by factors of 166 and 468 at pH 5 and 6 (see Ohman et al., J. Chem. Soc., Dalton Trans. (1983), p. 2513).
Other methods for cleaning metal and metal oxide residues on wafers include spraying water vapor into the plasma ashing chamber followed by introducing fluorine containing gases (hydrofluoric acid; U.S. Pat. No. 5,181,985, the disclosure of which is hereby incorporated by reference hereto) or a liquid containing hydrofluoric acid, ammonium fluoride and water with a pH from 1.5 to less than 7.
One method employing a fluoride-based chemistry is commonly assigned U.S. Pat. No. 6,248,704, the disclosure and figures of which are hereby incorporated by reference hereto for all purposes.
There is also a need to remove particulate residues from the wafer surfaces during the BEOL process. Currently most processes rely on an isopropanol and/or deionized water rinse with/or without ultra- or mega-sonic cleaning. For post chemical mechanical polishing (CMP) cleaning of oxide wafers, normally only deionized water is used, but with tungsten wafers a combination of dilute NH4OH and dilute HF usually are required.
There are five potential mechanisms for removing impurities (particles and/or ions) from the wafer surfaces:                1. Physical desorption by solvents, which involves replacing a small number of strongly absorbed particles with a large volume of weakly adsorbed solvent (changing the interaction of the surface charges);        2. Changing the surface charge with either acids or bases, e.g., making the Si—OH group positive, protonating with acid, or making the group negative with bases by removing the proton;        3. Ion complexation by removing adsorbed metal ions by adding acid (e.g., ion exchange);        4. Oxidation or decomposition of impurities, which involves oxidation of metals, organic materials or the surface of slurry particles, which changes the chemical forces between the impurities and substrate surface (the chemical reaction can either be through redox chemistry or free radicals); or        5. Etching the oxide surface, which releases the impurity while dissolving a certain thickness of the substrate surface.        
Currently available fluoride-based chemistries can help in items #2 and #5, but the cleaning conditions must be carefully controlled. In many cases, the components of the cleaning compositions are relatively toxic reactive solvent mixtures and thus must be subject to stringent use conditions and require hazardous chemical handling procedures and wearing of safety garments and apparel by users so as to avoid contact with the cleaning compositions. Additionally, because many of the toxic components of such cleaning compositions are highly volatile and subject to high evaporation rates, they require special human and environmental safety precautions to be taken during storage and use of the compositions.
Accordingly, there exists a need to develop improved cleaning compositions to efficiently clean a variety of deposits from a wide variety of substrates. Particularly in the field of integrated circuit fabrication, it should be recognized that the demands for improved cleaning performance with avoidance of attack on the substrates being cleaned are constantly increasing. This means that compositions that were suitable for cleaning less sophisticated integrated circuit substrates may not be able to produce satisfactory results with substrates containing more advanced integrated circuits in the process of fabrication.
These compositions should also be economical, environmental friendly and easy to use.
The present invention teaches such a new and improved cleaning composition and a process for its use. This composition is aqueous, dissolves both organic and inorganic substances, and, when used in the process, is able to clean a variety of substrates. The composition provides a more effective cleaning of the substrates, which means more effective residue removal, which in turn means that higher product yields can be obtained from the substrates being cleaned.